Elements of Information Retreival

Information retrieval is a process of accessing relevant information from a given data source. However the item of information or unit of information the user might be interested may vary from user to user and time to time.

Document
Historically of course, information retrieval generally meant 'retrieving' of a relevant document. For example document might be a research paper or a patent or government gazette. However with the unprecedented growth in the volume and variety of data available in digital format is changing the nature "retrieval" documents. And even the definition of document is undergoing a drastic change.
Query
Information retrieval process is initiated by the user by launching a 'query' to an IR system. The query could consists of a word or a phrase. Capturing query and converting that into a format for searching within the data source is the first job of IR system and this activity itself is a major chunk of work in an IR system.

Query processing IR system checks the entire source for any document(s) that matches the query and returns all such matched documents back to user. Such a list of documents is called "Searched result"
Relevance
User expects the search results to be useful for him. The extent to which the search results matches the expectation of the user is called " Relevance". The success of IR system hinges on its ability to get more and more "relevant" document for every user.